Recent years have seen major innovations in plywood manufacturing to increase productivity and decrease costs. One result of these innovations has been a shift of the process bottleneck to the drying operation. The need to increase drying throughput and the opportunity to improve product quality by reducing de-grade due to over-drying, has stimulated the development of adhesives tolerant to high veneer moisture. The results of research in this direction have not yet shown up in the U.S. patent literature, with the exception of U.S. Pat. No. 2,620,288.
The use of phenolic adhesives is well established in the plywood industry. Numerous formulations for such adhesives have been disclosed in the prior art, such as those described in the U.S. Pat. Nos. 2,150,693; 2,360,376; 2,437,981 and 2,457,493. These adhesives are generally applied as liquid resins. Phenolic adhesives are also well established in the waferboard industry, such as those described in U.S. Pat. Nos. 4,424,300 and 4,433,120. These adhesives are commonly applied as dry powders.
While such known adhesives are satisfactory for gluing dry veneers having moisture contents of 0% to 6%, they cannot be reliably employed with wet veneers having moisture contents as high as 10% or higher. With the latter, blisters develop on hot pressing, or even complete failure to form a bond under the usual conditions of temperature and pressing time which are employed in plywood manufacturing. In the case of waferboard, if the moisture content of the wafers is high, e.g. 10% or higher, excessive penetration of the adhesive into the wafers adversely affects bond quality.
Every plywood adhesive has a definable operating window with respect to the moisture available at the glue line. If insufficient moisture is present at the glue line, the adhesive is too dried out to flow adequately and form durable adhesive bonds. If too much moisture is present, excessive adhesive flow occurs leading to over-penetration of adhesive into the veneer and starved glue lines. When attempting to glue high moisture veneers, excessive flow and penetration must be overcome and further, a faster cure is required to overcome the inhibiting effect of the moisture on cure. In the present disclosure, resins have been formulated which show fast cure and good bond quality when high moisture veneers are incised and glued with the adhesive disclosed.
A primary object of the present invention is to provide an adhesive which can be employed successfully for gluing wet veneers or wafers having a higher moisture content than can be tolerated in present mill practice with commercial adhesives presently available. The higher moisture content is by way of example in the range of 10% to 20%.
A further object of the present invention is to provide a satisfactory adhesive for use both with dry and wet veneers and wood wafers to produce plywood, waferboard and composite board of quality equal to commercial specifications.
A further object of the present invention is to provide a phenol formaldehyde glue formulation that does not require a filler, requires a relatively short press time, and is capable of bonding veneers, wafers or pieces of wood without extensive adhesive loss from the glue line due to bleedthrough.
A further object is to provide a phenolic resin adhesive that is effective for hot press forming of plywood composite board and the like, with a relatively short cure time and which is effective on wet veneers.
A still further object of the present invention is to provide a composite board, waferboard, plywood panel or the like, bonded by an adhesive of the present invention.
A still further object of the present invention is to provide a composite board, plywood panel or the like, where the veneer or other starting material which has a relatively high impermeability to gas is improved by incising and where the veneers or other starting materials have a high moisture content.
A still further object of the present invention is to provide a plywood panel bonded by an adhesive of the present invention wherein the veneers having moisture contents higher than 8% are incised prior to hot press gluing.